The present invention relates to a ribbon to be used for wrapping, possibly for insulating purposes, and to be made of a synthetic material which is highly resistive against high temperature.
Ribbon of the type to which the invention pertains are known, for example, for electrically insulating electrical conductors. U.S. Pat. No. 3,408,453 describes such a ribbon to be made for example of polytetrafluoroethelene or PTFE for short. Such ribbons may be wrapped, for example, helically and directly upon electrical conductors. The outwardly facing side of such ribbons are for example, bonded upon a second lay of such a ribbon or one may use welding for interconnecting these ribbons. Basically, of course, the ribbon has a rectangular cross section. Particularly in the case of insulation complete coverage of the electrical conductor is necessary and this can be insured only if the ribbon overlaps in adjacent loops. This, however, introduces inherently a certain unevenness, the wrapped surface is no longer smooth but exhibits steps. A point to be considered in this regard, is that the step configuration introduced by the ribbon overlap establishes points on which external forces can act and if these forces are not exactly in radial alignment with the cylindrical configuration, force components may exist tending to tear the insulation off. It can readily be seen that this possibility constitutes a danger, particularly in those cases when the conductor is used for heating purposes or for measuring or tests purposes. Considering such an environment, a cable or conductor insulated in the described fashion may be subjected to high external temperatures, as well as to moisture or even chemically aggressive fluids. The more severe these environmental conditions are, the more crucial it becomes to have a smooth and homogenic outer surface of the insulated conductor so that any gap and open path to the conductor proper will not exist nor will be created with certainty.
For the known cable one can obtain these protective features and aspects by wrapping a particularly constructed ribbon around the polytetrafluoroethelene ribbon layer and bond or weld this additional ribbon thereto. Of course, this constitutes an additional expenditure, and does not eliminate the basic problem outlined above, namely the formation of edges or steps at overlapping ribbon loops. Thus, one may say that possibly any damage through mechanical wear is delayed at best. A first of one layer may be torn open with impunity, but the layer underneath now being exposed can be subjected to tear at a later time. Therefore, thus wrapping more layers of ribbon around the basic conductor does not eliminate the problem, it merely may delay the occurrence of damage.
In order to avoid these difficulties, it was deemed necessary to avoid the technique of ribbon wrapping and to insulate an electrical conductor directely through extrusion or the like in a continuous process, thereby producing indeed a smooth surface of continuous insulation. The procedure is analogous to jacketing a tube. This method is widely used particularly for applying a layer of thermoplastic or elastomeric material to a conductor, the material having a relatively low melting range. The extrusion and method of continuously applying a layer to the conductor is, however, not suitable if the layer material has a higher melting range, because the forming process becomes too difficult. One has instead used a sintering method for making compact insulation, but here is a decisive disadvantage that the material particles are orientedly predominantly in axial direction and that limits utilization of conductors insulated in this fashion. Moreover, sintering methods frequently does not establish a completely concentric arrangement of conductor and layer. Another disadvantage of the sintering method is that the length of the conductors to be treated in this manner is limited because the machinery providing the requisite pressing are comparatively small. Also it was found that particularly in the case of extruding a single layer insulation may produce occasionally local defects. Of course, extrusion is limited to the processing of concentric conductors.
The various considerations outlined above with reference to electrical cable and conductor are also applicable in many instances for the making of tubes. A particular field of employment are, for example, heat exchanger tubes for conducting aggressive media as disclosed, for example in German printed Pat. No. 8,106,819 using as basic material a metal tube, for example a copper tube and being jacketed with a fluoride containing synthetic such as the above-mentioned polypetrafluoroethelene. These jackets are made individually in forms of hoses in a so called piston extruder. Again, the material texture is predominantly oriented in longitudinal direction, and in the case of alternating thermo loads longitudinal cracks may readily occur in the wall of the hose. Such cracks, of course, may ultimately damage the heat exchanger to such a extent that it is no longer usable. This is even more important if the heat exchanger is used for heating aggressive electrolytic or strongly oxidizing acids.
The so called piston extrusion permits also the making of protective hoses from polytetrafluorethelene, but the piston extrustion can also be used to make tubes directly to be used for conducting a fluid medium. See for example German printed patent application No. 3,104,037. Aside from the fact that just as in the case of cable and conductor, the length to be processed in one run is limited it was found difficult to process tubes of larger diameter in this manner. In accordance with the known method, therefore, larger tubes are made from strips or parts of a number of smaller tubes which are joined through an internal mandral and bonded to each other in a melted state. For reasons above, this method is not suitable nor economically feasible for making PTFE tubes of larger cross sections and/or indefinite lengths.